This invention is related to a circuit for receiving data signals from an interface. More particularly, this invention is related to a circuit for extracting data which is transmitted using common-mode signals injected onto a differential data interface.
Differential signaling, in which two wires are used to transmit a differential data signal, is commonly used to transmit data in noisy environments where single-ended data signaling, using only one wire per data signal, cannot provide a sufficient degree of noise immunity. Environments of this type are particularly common in high performance systems which have reduced signaling levels to increase frequency.
In differential signaling, data is transmitted as the difference of the voltage or current on two differential signal components. When noise is present on the interface, the signal levels on the two differential signal components are affected substantially the same amount, and therefore the difference between the two signals remains unchanged. A significant drawback to this type of signaling, however, is that two wires are necessary to transmit each data signal. This is a particularly serious problem when providing communication between integrated circuits because the input and output wires physically limit the connectivity to the chip.
One newly discovered method to increase the amount of digital data carried on a differential interface is to utilize the common mode level of the differential signal as an additional data carrier. This method is described in a related U.S. patent application, entitled xe2x80x9cMethod and System of Data Transmission Using Differential and Common Mode Data Signaling,xe2x80x9d filed on the same day as the present application. According to the new technique, a first data signal is transmitted as differential signal components over a differential interfaces, such as a balanced pair of wires. A second data signal is transmitted by varying the common mode signal level carried by the differential signal pair. The common mode signal is injected in a manner which affects each component of the differential pair equally, thus preserving the difference between the two signals, and thereby the differentially transmitted data.
Several differential pairs may be used. For example, a first common mode signal may be injected into one differential pair and a second common mode signal may be injected onto a second differential pair. The two injected common mode signals are used to transmit differential signal components of a third data signal. Yet a fourth common mode data signal can be transmitted by raising or lowering the two common mode signals in a manner which does not affect the differentially transmitted, common-mode carried third data signal. In an alternative embodiment, one differential pair is used to send a differential signal and a common mode signal and a third wire is used to send a signal which is differentially varied with respect to the transmitted common mode signal to thereby transmit the common-mode data in a differential manner. In this alternative embodiment, two xe2x80x9cdifferentialxe2x80x9d signals are sent over three wires.
To extract data transmitted in this manner, it is necessary to determine both the differentially transmitted data signal and the common mode signal level. When a data signal is sent as a pair of differentially varied common mode signals, it is also necessary to extract the differentially transmitted data from the appropriate common mode signals. It is advantageous to be able to extract this type of data signal in a single circuit, instead of extracting each common mode signal individually, and then processing the extracted signals as conventionally transmitted differential signals.
In addition, it is advantageous to extract the common mode data signal without drawing a noticeable amount of current from the differential interface. This is particularly useful, for example, when the combined differential and common mode data transmitters and receivers are configured in a bus configuration having several receivers connected to the same data interface. Drawing current would affect the differential and common mode data levels and require the output drivers to source additional current for each receiver on the interface.
According to the invention, a common mode receiver circuit is provided which extracts a common mode data signal injected onto a differential interface. The differential signal components are applied to a common mode extraction unit with an impedance that is a function of the common mode signal carried on the signal components, but which is substantially independent of the specific differential signal values. By measuring the impedance, in the form of current flow or a voltage drop, the value of the common mode data signal can be extracted. In the preferred embodiment, the common mode extraction unit comprises two sets of substantially matched transistors, where each set has substantially the same overall operating characteristics. In the particular case where the transistors from each set have the same length, the sum of the device widths in each set is substantially equal. Each set is connected in parallel to a common current source and sink, where each transistor of the set is biased by an applied differential signal component.
In a second embodiment of the invention, a receiver circuit is provided to extract a differential data signal having differential signal components transmitted as common mode signals carried over a differential interface. The circuit includes two common mode extraction units connected in parallel to a common current source or sink in the form of a hybrid differential-style amplifier. The two differential components from the first differential interface are applied to the first unit and the two components from the second differential interface are applied to the second unit. According to the invention, each amplifier leg carries a current proportional to the common mode signal level on the respective differential pair, even as the magnitude of the applied differential signal components vary. Because the total current carried by the two legs is fixed, the magnitude of current carried by one leg indicates the value of a data signal differentially transmitted over the common mode signals.